Thermally-insulated anti-clog tip for electrocautery suction tubes

ABSTRACT

A suction coagulator includes an improved anti-clog tip. The anti-clog tip is located at the distal end of a hollow, conductive tube, the proximal end of which is connected to a handle. An interior channel runs through the conductive tube and the handle through a suction fitting thereon to a conventional source of suction. The handle also includes an electrical connection for providing electrical power to the conductive tube. An insulating layer surrounds the exterior sidewall of the conductive tube and is stripped back a distance D O  of 0.050 to 0.200 inches from the distal end of the conductive tube. A thermal insulating sleeve or coating is located inside of the interior channel of the conductive tube and extends from the distal end a distance D I  into said interior channel. The distance D I  is approximately 11/2 to 3 times the outside diameter OD C  of the conductive cannula tube. The insulating sleeve provides substantial thermal insulation and some electrical insulation. In a preferred embodiment of the invention, the distal end of the tube is flared into a bell creating an air gap having a width of D G  between the insulation sleeve and the inside diameter ID B  of the bell. The outside diameter of the bell is approximately equal to the outside diameter OD C  of the hollow, conductive cannula tube.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a suction electrocoagulator apparatus having ananti-clogging tip which includes a thermally insulating sleeve thatextends from the distal end of the hollow, conductive cannula tube intothe interior cavity of said tube.

2. Description of Related Art

The coagulation of bleeding blood vessels using electrically conductivesuction tubes, also referred to as cannulas, is a technique widely usedfor over two decades. A combined electrocautery and suction tubeinstrument is typically employed in surgery wherever excessive bloodmust be removed from the bleeding site in order to facilitate hemostatof bleeding vessels using the electrocautery feature of the instrument.

Commercially available suction coagulators made expressly forelectrocautery hemostat procedures generally have the following commoncomponents:

(A) A hollow, metallic, conductive cannula tube having a suitablethickness of non-conducting electrical insulation on its exterior. Theelectrical insulation is absent from the first few millimeters on thetube's distal end in order to form an annular ring for electrocauteryprocedures. The sole purpose of the insulation is to protect the patientand doctor from cautery burns that would result from contact with theoutside of the metallic tube when it is energized.

(B) A non-metallic or electrically insulated handle. The handle includesa suction cavity or passageway running through its length thatterminates in a suction fitting at one end for connection to a suctionsource. At its proximal end the handle is hermetically coupled to thehollow, metallic tube so that the suction cavity of the handlecommunicates directly with the suction channel of the tube. The handlemay also have a venting passageway that connects the main suction cavitywith a vent hole on the handle's exterior. The surgeon uses fingerpressure on the vent hole to control the amount of suction applied.

(C) A power cord, typically in an insulated wire, is used to connect thesuction coagulator to a high frequency current generator. The power cordenters the handle and is electrically connected to the proximal end ofthe conductive suction tube using various known connecting techniques.

One of the major problems with electrocautery suction devices is thatthe tips tend to clog with burned tissue and coagulated blood. Intheory, the suction is supposed to remove the clog but the suctionitself produces some problems.

One approach has been to control the flow of air through the suction tipby means of a vent hole that can be manipulated by the surgeon. A usefuldescription of the prior art problems is found in my prior U.S. Pat. No.4,932,952 entitled ANTISHOCK, ANTICLOG SUCTION COAGULATOR. The followingpatents were cited as being relevant to that approach: U.S. Pat. Nos.2,888,928; 3,595,234; 3,610,242; 3,828,780; 3,857,945; 3,974,833;4,427,006; 4,562,838; and 4,719,914.

Another approach to minimize the coagulating of tissue in anelectrocautery suction instrument is described in U.S. Pat. No.3,902,494 entitled SUCTION SURGICAL INSTRUMENT. The general conceptdescribed therein provides for an electrode at the tip of the instrumentlocated in such a way as to impede the introduction of tissue mass intothe tube. By restricting the size of the opening with the conductingelectrode, it is possible to filter out tissue that otherwise might clogthe instrument downstream. One embodiment described in the text of U.S.Pat. No. 3,902,494, but not illustrated in its drawings, comprehends anelectrical lead shaped as a tube which encompasses the suction shaft 10.The purpose appears to be to increase the stiffness of the suctionshaft. In that embodiment, it is assumed that the electrode tip of thesuction device is likewise restricted to prevent tissue clogging astaught by the other illustrated embodiments of the invention. Insofar asunderstood, U.S. Pat. No. 3,902,494 is directed at the concept ofpreventing tissue clogging and does not appear to address the moredifficult issue of clogging caused by the coagulation of blood at themouth of the cannula due to the heat generated by the electrocauteryeffect.

U.S. Pat. No. 4,682,596 entitled MEDICAL HIGH FREQUENCY COAGULATIONINSTRUMENT describes a specialized structure incorporating an insulatinghose inside of a ring electrode which is part of a much more complicatedoverall structure involving a second ring electrode and an exteriorinsulating layer as well as an intermediate insulating layer. Thepurpose of the device appears to provide a supply of "flushing liquid"which emerges around the interior electrode and then is sucked downthrough the central shaft of the interior electrode.

U.S. Pat. No. 4,682,596 and its counterpart U.S. Pat. No. Re. 22,925both describe a catheter employing an electrode and an electricallyinsulating tube. In that embodiment, the electrode appears to cover theentire front face of the device as illustrated in FIGS. 3 and 4 thereof.There is a discussion in column 8, lines 44-47 that addresses theinsulating properties of the support for the electrode tip.

Lastly, U.S. Pat. No. 4,347,842 entitled DISPOSABLE ELECTRICAL SURGICALSUCTION TUBE AND INSTRUMENT is cited as being of general possiblerelevance only.

While the prior art selectively discloses the concept of restrictingtissue from entering the tip of an electrocautery tube so as to preventclogging of the tube, nevertheless, it does not appear to appreciate theproblem of blood coagulation at the tip due to electrocautery heating.Accordingly, none of the prior art located and described above appearsto fully appreciate the necessity of providing appropriate thermalinsulation at the tip in order to avoid blood coagulation. The inventiondescribed in detail later in this specification, provides a dramaticimprovement over prior art devices such as described above by virtue ofthe fact that it substantially eliminates the problem previouslyassociated with the formation of eschar, i.e. blood char, in the tip ofelectrocautery suction devices.

SUMMARY OF THE INVENTION

Briefly described, the invention comprises a suction electrocoagulatorapparatus having an improved anti-clog tip. The anti-clog tip is locatedat the distal end of a hollow, conductive, cannula tube. The cannulatube includes an exterior sidewall, the distal end, a proximal end atthe opposite end of the tube from the distal end, and an interiorchannel for connecting the proximal and distal ends together. Theproximal end of the conductive tube is connected to the front end of ahandle. A suction fitting is located on the rear end of the handle andis employed to communicate the interior cavity of the conductive tube toa source of suction. The handle also includes an electrical connectorfor providing high-frequency current to the tip of the conductive tube.Electrical insulation, in the form of a coating or sleeve, surrounds theexterior of the conductive tube and is stripped back by approximately0.050 to 0.200 inches from the distal end of the conductive tube.

The improvement comprises a thermal insulating sleeve located inside ofthe interior channel of the conductive tube and extending a distanceD_(I) into the interior channel. The distance D_(I) is preferably in therange of one and one-half to three times the outside diameter of thehollow, conductive, cannula tube. The insulating sleeve preferablyprovides both electrical and thermal insulation. The insulation cancomprise either a combined sleeve or two discrete sleeves, one forthermal insulation and one for electrical insulation, locatedconcentrically within each other.

An alternative embodiment of the invention calls for flaring the distalend of the electrically conductive, hollow, metallic, cannula tube intoa bell, the outside diameter OD_(B) of which is larger than the outsidediameter OD_(I) of the cannula tube and the inside diameter which islarger than the downstream inside diameter ID_(C) of the conductivetube. An insulator providing both electrical and thermal insulation islocated on the inside of the bell and includes a channel therethroughthat has a diameter ID_(S) that is approximately the same as the insidediameter ID_(C) of the conductive tube.

The preferred embodiment of the invention calls for flaring the tip ofthe hollow, metallic, conductive cannula tube into a bell having anoutside diameter OD_(B) approximately equal to the outside diameterOD_(I) of the insulating coating on the outside of the conductive tube.A cylindrical, insulating sleeve is inserted into the distal end of theanti-clog tip so that it extends a small distance D_(S) beyond the belland a distance D_(I) into the throat of the anti-clog tip. An annulargap having a width D_(G) is formed between the outside surface of thecylindrical insulating sleeve and the inside surface of the bell. Theair gap between the insulating sleeve and the inside surface of the bellprovides additional thermal insulation. Even though the cylindricalinsulating sleeve slightly restricts the inside diameter of the cannulatube, nevertheless, the results are significantly improved because ofthe additional thermal insulating provided by the air gap between theinsulating sleeve and the inside surface of the bell.

These and other features of the present invention will be more fullyunderstood by reference to the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a top plan view of the invention including the anti-clog tip.

FIG. 1B is a side elevation view of the embodiment of FIG. 1A.

FIG. 1C is a side cross-sectional view of the embodiment of FIGS. 1A and1B.

FIG. 2 is a side, cross-sectional view of an early, but relativelyineffective, prototype of the present invention illustrating a thinpolytetrafluoroethylene coating extending from the distal end of the tipinto the interior channel of the conductive cannula tube.

FIG. 3 illustrates an improved embodiment of the present inventionincluding a combined or integrated electrical and thermal insulatingsleeve that extends a distance D_(S) beyond the distal end of theconductive cannula tube and a distance D_(I) into the throat of thecannula tube.

FIG. 4 illustrates another embodiment of the present invention employingseparate electrical and thermal insulating sleeves, concentricallypositioned, as opposed to the integrated sleeve illustrated in FIG. 3.

FIG. 5 illustrates another embodiment of the present invention in whichthe conductive, cannula tube is flared into a bell at its distal end andin which the insulating sleeve is positioned within the bell such thatthe inside diameter of the apparatus from the bell section through theconductive tube remains essentially unchanged.

FIG. 6 is a cross-sectional view of the preferred embodiment of theinvention in which the conductive tube is flared into a bell at itsdistal end as in FIG. 5 and in which an insulating sleeve is locatedwithin the throat of the tip creating a thermally insulating air gapbetween the exterior surface of the cylindrical insulating sleeve andthe interior surface of the bell.

DETAILED DESCRIPTION OF THE INVENTION

During the course of this description like numbers will be used toidentify like elements according to the different figures whichillustrate the invention.

FIGS. 1A, 1B and 1C, illustrate different views of the present invention10 in which the improved, anti-clog tip 20 is shown in its normalcontext. The invention 10 typically includes an electrical conductor 12,which is connectable to a conventional source of high frequency current,a handle 14, a vent hole 16, and cannula tube 22. Cannula tube 22 issurrounded by an electrically insulating coating 18 which is strippedback a distance of D_(S) from the distal end of the hollow, conductive,metallic cannula tube 22. The proximal end of the cannula tube 22 isconnected to the front end 28 of handle 14. A suction fitting 24 islocated at the rear of handle 14 and connects with a hollow interiorcavity 26 which communicates with the interior cavity and opening 30 ofthe cannula 22. Handle structures that might be suitable for use withthe present invention 10 are described in the prior art. See, forexample, U.S. Pat. No. 3,828,780 and my U.S. Pat. No. 4,932,952previously discussed.

FIGS. 2-6 illustrate various different embodiments of the improvedanti-clog tip 20 in further detail.

According to FIG. 2, the electrically conductive, tubular cannula 22extends a distance D_(O) beyond the exterior insulating sleeve 18.Conductive tube 22 has a central axis 32 and a front opening 30 whichdraws in air and blood under the influence of suction applied to suctionfitting 36 as illustrated in FIGS. 1A-1C. The embodiment 40, of FIG. 2,includes a front face 44 and a thin interior Teflon® sleeve 42 whichextends a distance D_(I) from the front face 44 into the interior of theelectrically conductive tube 22. The purpose of sleeve 42 was to provideelectrical insulation but not thermal insulation. The distance D_(I)extended approximately 11/2 to 3 times the diameter of the tube into thesuction channel. For tonsillectomy procedures the interior diameter ofthe tube is typically 0.100 inches, and the outside diameter of the tube22 is typically 0.120 inches and, therefore, the distance D_(I) mightvary between 0.150 to 0.360 inches. While the electrical insulationembodiment 40 of FIG. 2 works (with proper sizing of the Teflon® sleevethickness), it does not provide the dramatically improved level ofresults achieved with embodiments 3-6, described below.

Embodiment 50, illustrated in FIG. 3, includes a tubular electrode 22,which extends a distance D_(O) beyond the outer electrical sleeve 18. Anelectrical and thermal insulating sleeve or coating 52 is located withinthe inside diameter of cannula 22 and extends a distance D_(S) beyondthe front face 44 of cannula 22. The distance D_(S) should be shortenough so that the electrical and thermal insulating sleeve 52 does notinterfere with the surgeon's ability to touch tissue with the cauterytip face 44. The minimum distance D_(I) that the sleeve 52 must extendinto the cannula 22 may be derived empirically but testing indicatesthat a distance of 11/2 to 3 times the outside diameter of the cannulatube 22 is adequate (i.e., D_(O) =11/2-3 OD_(C)). The thickness T_(S) ofthe sleeve 52 depends upon the physical properties of the thermalinsulating material selected. It must be thick enough to prevent thepassage of most of the heat of cautery while at the same time insulatingthe inside surface from most or all of the electrocautery current. Thedesign parameters for this are complex. The material of the sleeve 52must be biocompatible, have low thermal conductivity, high electricalresistivity, and be capable of withstanding temperatures up toapproximately 500° F. Good candidate materials for sleeve 52 wouldinclude, but not be limited to, glass, or high temperature plastics suchas Teflon® (polytetrafluoroethylene), polysulfone, polyethersulfone, orpolyetherimide. The preferred electrical resistivity of the sleeve isgreater than 10¹² ohm cm and preferably about 10¹⁶ ohm cm. For goodresults the material selected should have a thermal conductivity lessthan 0.6 . While the foregoing thermal conductivity is good, theapparatus also operates satisfactory with a sleeve thermal conductivityof approximately 0.10 .

According to the preferred embodiment, the cannula tube 22 is preferablymade from an electrically conducting material such as brass, aluminum orsteel. The basic electrical insulation 18 on the outside of the cannula22 is preferably polyolefin but could be PVC, PTFE or any otherflexible, suitable plastic material. The insulation 18 is preferablystripped back a distance D_(O) of 0.05 to 0.200 inches from the barecautery tip face 44 of the cannula 22.

FIG. 4 illustrates an embodiment 60 in which the insulating sleeve cancomprise two concentric sleeves 62 and 64. According to the preferredembodiment, the innermost sleeve 64 comprises an electrical insulatingmaterial preferably PTFE, FEP, PFA, or polyetherimide. Its thicknessT_(E) is preferably 0.010 and in the range of 0.003 to 0.020 inches.Thermal insulating sleeve 62 surrounds the electrical insulating sleeve64 and has a thickness T_(T) of approximately 0.010 and could be in therange of 0.005 to 0.020 inches. The preferable material for the thermalinsulating sleeve 62 is filled epoxy but could be filled silicone orfiberboard.

An improved alternative embodiment 70 is illustrated in FIG. 5.Embodiment 70 includes a modified tip that preserves the inside diameterID_(C) while utilizing the basic design principles of the presentinvention to prevent blood coagulation due to thermal heating. Accordingto embodiment 70, the end of the conductive cannula tube 22 is flared toform a bell 72. The outside diameter OD_(B) of the bell 72 is largerthan the outside diameter OD_(I) of the insulating sleeve 18. It ispossible with this structure to place an electrical and thermalinsulating sleeve 74 into the mouth of the bell 72 in such a way thatthe inside diameter ID_(S) of the sleeve 74 is approximately the same orgreater than the downstream inside diameter ID_(C) of the conductivecannula tube 22. According to the tip embodiment 70, the sleeve 74 couldbe a homogenous material having both electrical and thermal insulatingproperties, such as illustrated by sleeve 52 in the embodiment 50illustrated in FIG. 3. Alternatively, the sleeve 74 could be comprisedof two separate concentric sleeves, one for thermal insulation and theother for electrical insulation, such as sleeves 62 and 64 illustratedin embodiment 60 of FIG. 4. This embodiment, however, requires anawkwardly large outside diameter of the bell in order to accommodatesufficient thickness for the electrical and thermal insulating sleeve(s)74. Another version of embodiment 70 would be to reduce the outsidediameter OD_(B) of the bell while making the insulating sleeve insidediameter ID_(S) somewhat less than the downstream inside diameter ID_(C)of the conductive cannula.

The preferred tip embodiment 80 of the invention 10 is illustrated inFIG. 6. This tip was the most successful of those used in field trials.It was especially useful for the type of electrocautery suction tubesthat are used in tonsillectomy, adenoidectomy, or sinuscopy procedures.According to the preferred embodiment 80, it is desired to use abell-shaped tip 72, such as illustrated by embodiment 70 of FIG. 5, but,because of surgeon's preference, to limit the outside diameter of thebell OD_(B) to approximately match the outside diameter OD_(I) of theexternal insulation 18. An electrical/thermal insulating sleeve 82having an inside diameter ID_(S) is located in the opening 30 of theinstrument and extends a distance D_(S) beyond the front face 44 of thebell 72 and has a length L_(S) sufficient so that it extends into, andis held in place by, the sidewalls of the upstream cannula tube 22. Theexterior wall of the insulating sleeve 82 defines an air gap 84 withrespect to the interior wall of the flared, bell section 72 of thecannula tube 22. The annular air gap 84 formed between the sleeve 82 andthe bell 72 acts as a very good thermal insulator so that the requiredthickness of the insulating sleeve 82 is largely reduced to the minimumrequired for electrical insulating properties. While it is possible tofill the gap 84 with a thermal insulator, it is has been found that theair gap 84 provides more than adequate thermal insulation with a sleeve82 having a thickness T_(S) of approximately 0.010 inches fortonsillectomy size suction tubes.

EXPERIMENTAL RESULTS

The results achieved by using the basic invention, especiallyembodiments 70 and 80, illustrated in FIGS. 5 and 6, were exceptionallygood. For years electrocautery suction tube designers have attempted tominimize clogging by modifying the port designs such as described in myprior U.S. Pat. No. 4,932,952. While the modified port designs improvedthe situation, they did not remedy the underlying problem, namely thatthe tips would still clog. Accordingly, tests were initiated to try todetermine the root causes for tip clogging.

It was generally thought, prior to the time of the present invention,that the cause of rapid coagulation of blood within the tip ofelectrocautery suction tubes was the direct result of the fulgurationeffects of the electrocautery current discharged at that location.Therefore, my first approach was to attempt to electrically insulate theinside diameter and tip end with a 0.006 inch coating ofpolytetrafluoroethylene (i.e., Teflon®) as illustrated in embodiment 40of FIG. 2. The coating 42 extended a distance D_(I) of approximately 1/2inch into the tube 22 from its tip 44 and its thickness essentiallyprecluded the conductance of current in the coated area throughresistive or capacitive means. The concept was to drive all the cauterycurrent to the outside of the tip 40. Embodiment 40 was tested at theNorth Carolina State University College of Veterinary Medicine usingfresh drawn horse blood. Suction tubes having tips with and without thecoating 42 were each placed into a 100 ml plastic cup filled with horseblood. The ESU generator was set at 50% coagulation power and activatedwith moderate suction applied to the suction port. Without exception,the uncoated samples clogged within 10 to 20 seconds and the coated onesdid not. Moreover, the inside diameters of the coated samples werecompletely clean.

Initially, it was believed that embodiment 40, illustrated in FIG. 2,solved the problem, but it was later discovered through actual use intonsillectomy procedures that those tubes clogged almost as often as theuncoated ones.

It was then observed that the laboratory tests did not simulate actualuse of the device in one important aspect, namely, that in thelaboratory the tip of the cannula was always submerged in blood that wasbeing drawn up into the cannula. The circulating blood kept the tip coolby removing the heat generated by cautery. In actual use, however,suction is used first to remove most of the blood to visualize thebleeder area so that when cautery is applied only a mixture of air and asmall amount of blood passes up the cannula to cool the tip. Therefore,the laboratory protocol provided much more liquid cooling of the cauterytip than was found in actual use. This observation led correctly to thespeculation that the coagulation of blood within the cautery tip couldbe minimized if the cautery tip was insulated inside the diameter fromboth the electrocautery current and the heat it generated. The resultingtip is a structure that easily and economically prevents the clogging ofelectrocautery tips from both tissue and blood in a safe, dependablemanner.

While the invention has been described with reference to a preferredembodiment, it will be appreciated by those of ordinary skill in the artthat modifications can be made to the structure and function of thebasic invention without departing from the spirit and scope of theinvention as a whole.

I claim:
 1. A suction coagulator apparatus having an improved anti-clogtip, said apparatus comprising:a hollow, conductive tube having anexterior sidewall, a distal end, a proximal end at the opposite end ofsaid tube from said distal end, and an interior channel therethrough forconnecting said proximal and distal ends; a handle having a front endattached to the proximal end of said tube, a rear end having a suctionfitting thereon, and an interior cavity for connecting said suctionfitting to the interior channel in said tube such that when suction isapplied to said suction fitting, air is drawn through the distal end ofsaid tube in a downstream direction through the channel in said tube andthen through said interior cavity to said suction fitting; electricalconnection means attached to said handle for making electricalconnection with said tube; an insulation coating surrounding theexterior sidewall of said conductive tube and stripped back a distanceD_(O) from the distal end of said tube; and, insulating means locatedinside of said interior channel of said conductive tube and extendingfrom said distal end a distance D_(I) into said interior channel,wherein said insulating means prevents clogging of blood inside of thedistal end of said tube.
 2. The apparatus of claim 1 wherein saidinsulating means comprises an electrical insulation sleeve.
 3. Theapparatus of claim 2 wherein the electrical resistivity of said sleeveis greater than 10¹² ohm cm.
 4. The apparatus of claim 3 wherein theelectrical resistivity of said sleeve is approximately 1×10¹⁶ ohm cm. 5.The apparatus of claim 1 wherein said insulating means comprises athermal insulation sleeve.
 6. The apparatus of claim 5 wherein thethermal conductivity of said sleeve is less than 0.6 .
 7. The apparatusof claim 6 wherein the thermal conductivity of said sleeve isapproximately 0.10 .
 8. The apparatus of claim 1 wherein said insulatingmeans comprises an electrical and thermal insulation sleeve.
 9. Theapparatus of claim 1 wherein said hollow, conductive tube has an outsidediameter OD_(C) and wherein the distance D_(I) is approximately 11/2-3times OD_(C).
 10. The apparatus of claim 1 wherein the material of saidinsulating means is taken from the group consisting of: glass, ceramic,or high-temperature plastic selected from the group consisting ofpolyfluorocarbon, polysulfone, polyethersulfone, polyamide,polyetherimide, silicone, or epoxy.
 11. The apparatus of claim 1 whereinsaid insulation coating surrounding said exterior sidewall of saidconductive tube is stripped back from the distal end of said tube by adistance D_(O) that is approximately in the range of 0.050 to 0.200inches.
 12. The apparatus of claim 1 wherein said conductive tube has adownstream inside diameter ID_(C) and wherein said distal end of saidtube is flared into a bell having an interior surface and an insidediameter ID_(B) greater than the downstream inside diameter ID_(C) ofsaid conductive tube.
 13. The apparatus of claim 12 further comprisingan air gap having a width D_(G) between said insulating means and saidinterior surface of said bell.
 14. The apparatus of claim 13 whereinsaid exterior insulation coating has an outside diameter of OD_(I) andwherein said bell has an outside diameter OD_(B) which is substantiallyequal to said outside diameter OD_(I) of said exterior insulationcoating.
 15. In a suction coagulator of the type having a hollow,conductive tube, including an exterior sidewall, a distal end, aproximal end at the opposite end of said tube from said distal end, andan interior channel therethrough for connecting said proximal and distalends, and a handle attached to the proximal end of said conductive tube,said handle including a suction fitting for mating with a source ofsuction and an electrical connection for providing electrical power tosaid conductive tube, the improvement comprising:an insulation coatingsurrounding the exterior sidewall of said conductive tube and strippedback a distance D_(O) from the distal end of said tube; and, insulatingmeans located inside of said interior channel of said conductive tubeand extending from said distal end a distance D_(I) into said interiorchannel, wherein said tip apparatus substantially eliminates the burningof blood and the clogging of the inside of the distal end of saidconductive tube by substantially preventing the blood in said tube fromheating up and coagulating during electrocautery procedures.
 16. Theapparatus of claim 15 wherein said conductive tube has a downstreaminside diameter of ID_(C) and wherein said distal end of said tube isflared into a bell having an inside surface and an inside diameter ofID_(B) greater than said downstream inside diameter ID_(C) of saidconductive tube.
 17. The apparatus of claim 16 further comprising an airgap between said insulating means and said inside surface of said bell,said air gap having a width of D_(G).